Behavioural,biochemical and electrophysiological studies on the motor depressant and stimulant effects of bromocriptine

Summary Bromocriptine (BRC) produced a biphasic behavioural effect in mice; an early depressant phase which lasted for about 1 h and a later stimulant phase which lasted from about 1 to 5 h. The stimulation was blocked with SCH23390. Both phases of activity were accompanied by marked striatal DA autoreceptor effects as indicated by reductions in dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) levels and by a reduction in the accumulation of DOPA (after inhibition of nigrostriatal DA nerve firing and DOPA decarboxylase). However, while the autoreceptor effects were still evident during the behavioural stimulant phase, there was a gradual rise in DOPAC and HVA from 1 to 4 h after injection, indicating a gradually increasing DA turnover. We were unable, using a variety of behavioural and biochemical paradigms, to demonstrate any change in DA autoreceptor sensitivity after one dose of BRC. In electrophysiological studies, however, it was found that prior exposure of rats to one dose of BRC rendered them subsensitive to the rate-inhibiting effects of a second dose of BRC, as measured in anaesthetized animals using extracellular single cell recordings of identified DA neurons in the substantia nigra pars compacta. It is concluded firstly, that the stimulant phase of BRC in mice occurs despite continued occupation of the DA autoreceptors by BRC because adequate endogenous DA is available to provide the required D1 receptor stimulation and secondly, that the terminal autoreceptors in the striatum (as assessed in mice using biochemical techniques) may be regulated differently to the somatodendritic autoreceptors (as assessed electrophysiologically in rats). Send offprint requests to: D. M. Jackson at the above address     

Withdrawal from repeated morphine sensitises mice to the striatal dopamine release enhancing effect of acute morphine

The effects of morphine withdrawal and challenge on the a-methyl-p-tyrosine (MT)-induced depletion of dopamine (DA) as well as on DA metabolism and ³H-SCH 23390 and ³H-spiperone binding were studied in the striata of male mice. Morphine was given s.c. 3 times daily for 5 days followed by 1 to 3 days' withdrawal.The MT induced DA depletion was retarded in mice withdrawn for 1 day from repeated morphine. At this time point the striatal concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) fell, too. In mice withdrawn for 3 days from morphine neither DA depletion nor DOPAC or HVA concentrations differed from those of control mice. In control mice acute morphine challenge accelerated the DA depletion at the dose 10 mg/kg but not at the dose 30 mg/kg. Both doses elevated striatal DOPAC and HVA. In mice withdrawn from repeated morphine for 1 day acute morphine partially counteracted the withdrawal-induced retardation of DA depletion and elevated striatal DOPAC and HVA clearly less than in control mice. However, in mice withdrawn for 3 days 10 mg/kg of morphine clearly enhanced DA depletion and its effect on striatal HVA was significantly augmented. In these mice as in controls the 30 mg/kg dose did not alter striatal DA depletion and elevated HVA less than in controls. Acute morphine did not alter striatal 3-methoxytyramine (3-MT) concentration in control mice but at the dose 10 mg/kg increased it in mice withdrawn for 3 days. Morphine withdrawal did not significantly affect striatal 3H-SCH 23390 binding, but slightly decreased ³H-spiperone binding in mice withdrawn for 3 days indicating a down-regulation of D₂ receptors.Our results by using three different indices of DA release (DA depletion after aMT, HVA and 3-MT) show that long enough withdrawal from repeated morphine treatment augments the morphine-induced release of striatal DA in mice. We propose that the striatal DA release in mice is regulated by two opposite opioid sensitive mechanisms with different dose-dependencies and different tolerance development. Correspondence to: L. Ahtee at the above address     

3- and 4-O-sulfoconjugated and methylated dopamine: highly reduced binding affinity to dopamine D2 receptors in rat striatal membranes

Summary The binding properties of 3- and 4-O-sulfoconjugated dopamine (DA-3-0-S, DA-4-0-S) as well as 3-O-methylated dopamine (MT) to rat striatal dopamine D₂ receptors were investigated. ³H-spiperone was used as a radioligand in the binding studies. In saturation binding experiments (+)butaclamol, which has been reported to bind to dopaminergic D₂ and serotoninergic 5HT₂ receptors, was used in conjunction with ketanserin and sulpiride, which preferentially label 5HT₂ and D₂ receptors, respectively, in order to discriminate between ³H-spiperone binding to D₂ and to 5HT₂ receptors. Under our particular membrane preparation and assay conditions, ³H-spiperone binds to D₂ and 5HT₂ receptors with a maximal binding capacity (B _max) of 340 fmol/mg protein in proportions of about 75%:25% with similar dissociation constants K _D (35 pmol/l; 43 pmol/l). This result was verified by the biphasic competition curve of ketanserin, which revealed about 20% high (K _D = 24 nmol/l) and 80% low (K _D = 420 nmol/l) affinity binding sites corresponding to 5HT₂ and D₂ receptors, respectively. Therefore, all further competition experiments at a tracer concentration of 50 pmol/l were performed in the presence of 0.1 mol/l ketanserin to mask the 5HT₂ receptors. DA competition curves were best fitted assuming two binding sites, with high (K _H = 0.12 mol/l) and low (K_L = 18 mol/l) affinity, present in a ratio of 3:1. The high affinity binding sites were interconvertible by 100 mol/l guanyl-5-yl imidodiphosphate [Gpp(NH)p], resulting in a homogenous affinity state of DA receptors (K _D = 2.8 mol/l). Competition experiments with various compounds confirmed the binding of ³H-spiperone to D₂ receptors. DA-3-O-S, DA-4-O-S, and MT were more than 5,000-, more than 10,000-, and 530-fold less potent in competing for ³H-spiperone binding when compared with DA at the high affinity binding site which mediates biological effects. Therefore, it is concluded that these DA metabolites are biologically ineffective at central D₂ receptors. Send offprint requests to E. Werle at the above address     

6-Hydroxydopamine and 5,7-dihydroxytryptamine selectively reduce dopamine and 5-hydroxytryptamine metabolites in cerebroventricular perfusates of rats

The efflux into the lateral cerebral ventricles of metabolites of dopamine (DA) and 5-hydroxytryptamine (5HT) was determined in unanesthetized rats bearing chronically implanted push-pull cannulae. Pretreatment with 6-hydroxydopamine (6-OHDA) reduced the basal efflux of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), but not of 5-hydroxyindoleacetic acid (5HIAA). The haloperidol-induced increase in the efflux of DOPAC and HVA was markedly attenuated in the 6-OHDA-pretreated rats. In rats treated with 5,7-hydroxytryptamine (5,7-DHT) the basal efflux of DOPAC and HVA was unaffected, while that of 5HIAA was markedly reduced; in these animals the ability of L-tryptophan to increase the perfusate content of 5HIAA was abolished. These results indicate that metabolites of DA and 5HT appearing in cerebroventricular perfusates of rats originate from DA and 5HT neuronal terminals in the brain.     

Parametric and pharmacological analyses of the enhanced grooming response elicited by the D1 dopamine receptor agonist SKF 38393 in the rat

The present report investigated several parametric and pharmacological aspects of the enhanced self-grooming behavior of rats following systemic administration of the selective D₁ dopamine (DA) receptor agonist SKF 38393. The amount of time that rats spent grooming themselves was measured continuously for 30 min following drug administration to provide a quantitative measure of the drug-induced behavior. SKF 38393 increased the amount of grooming in a dose-dependent manner (0.5–16 mg/kg, SC). The onset of this effect required at least 5 min and it persisted for at least 60 min. The ability of SKF 38393 to enhance grooming was shared by R-SKF 38393, but not S-SKF 38393, consistent with the affinities of these enantiomers for the D₁ DA receptor. Unlike SKF 38393, the peripheral D₁ agonist fenoldopam (SKF 82526) failed to cause an increased grooming response, suggesting a central site of action for elicitation of this behavior. The SKF 38393-induced increase in grooming was competitively antagonized by the D₁ selective antagonist SCH 23390 (0.5 mg/kg, SC). Although the D₂ DA receptor-selective antagonist eticlopride reduced SKF 38393-elicited grooming, this antagonism appeared to be of a physiological rather than pharmacological nature. When eticlopride was coadministered with the non-selective (mixed) D₁/D₂ agonist apomorphine, an increase in grooming behavior similar to that produced by SKF 38393 was observed. Inactivation of D₁ and D₂ DA receptors produced by pretreatment with the irreversible antagonistN-ethoxycarbonyl-2-ethoxy-1, 2-dihydroquinoline (EEDQ), at a dose which reduces D₁ and D₂ receptor density by 50% (8.0 mg/kg, IP), reduced SKF 38393-induced grooming by approximately 50%. Prior protection of D₁ receptors by SCH 23390 completely prevented the effect of EEDQ whereas prior protection of D₂ receptors by eticlopride did not. These results demonstrate that enhanced grooming behavior elicited by dopamine agonists in rats, when measured as the amount of time spent grooming, provides a reliable, quantifiable index of selective D₁ DA receptor activation in the CNS. In addition, this behavior does not appear to require concurrent stimulation of D₂ DA receptors by endogenous DA.     

The selective dopamine D2 receptor antagonist raclopride discriminates between dopamine-mediated motor functions

The actions on central dopamine (DA) mechanisms of raclopride, a new substituted benzamide, were studied by means of behavioural and biochemical methods in the rat. Raclopride blocked the in vitro binding of the dopamine D₂ antagonist ³H-spiperone (IC₅₀=32 nM), but not of the unselective D₁ antagonist ³H-flupenthixol (IC₅₀>100,000 nM) in rat striatum, and failed to inhibit striatal DA-sensitive adenylate cyclase in vitro (IC₅₀>100,000 nM). Raclopride caused a dose-dependent increase in the DA metabolites HVA and DOPAC in the striatum and olfactory tubercle. Behavioural studies showed that raclopride discriminates between the motor behaviours induced by the DA agonist apomorphine. Thus, unlike haloperidol, raclopride blocked apomorphine-induced hyperactivity at considerably lower doses than those inhibiting oral stereotypies. Moreover, raclopride showed a high separation between the doses for blockade of apomorphine-induced hyperactivity and those inducing catalepsy in rats. Raclopride caused a dose-dependent blockade of the specific binding of ³H-spiperone and ³H-N-n-propylnorapomorphine (³H-NPA) in vivo at doses similar to those blocking the behavioural effects of apomorphine. The maximal blockade of ³H-spiperone binding in vivo was lower for raclopride than for haloperidol. Raclopride caused a greater inhibition of ³H-NPA than of ³H-spiperone in vivo binding in the striatum. It is suggested that the ability of raclopride to discriminate between different DA-mediated functions may be attributed to a preferential blockade of a subclass of functionally coupled dopamine D₂ receptors in striatal as well as in extrastriatal brain regions in the rat.     

In vivo effects of some histamine H1-receptor antagonists on monoamine metabolism in the mouse brain

Summary The in vivo effects of four Hr-antagonists, diphenhydramine, chlorpheniramine, mepyramine, and promethazine, on the metabolism of noradrenaline (NA), dopamine (DA), and 5-hydroxytryptamine (5-HT) were investigated in the whole mouse brain. Diphenhydramine and chlorpheniramine had no significant effect on levels of NA, 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG), DA, and 5-HT, but they significantly decreased levels of 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA). In particular chlorpheniramine markedly decreased 5-HIAA levels at doses as low as 1 mg/kg, i. p. Mepyramine significantly decreased 5-HIAA levels but not those of other substances. High doses of promethazine significantly decreased NA levels but markedly increased those of MHPG, DOPAC, HVA, 5-HT, and 5-HIAA. The DA reduction induced by -methyl-p-tyrosine (-MT) was significantly inhibited by diphenhydramine, chlorpheniramine, and promethazine, but the -MT-induced NA decrease was significantly enhanced by promethazine. The 5-HIAA accumulations induced by probenecid were significantly inhibited by chlorpheniramine and mepyramine. These results suggest: (1) Diphenhydramine and chlorpheniramine inhibit the turnover of both DA and 5-HT by blocking their neuronal uptake. (2) Promethazine and mepyramine inhibit DA and 5-HT turnover, respectively, as a result of the inhibition of the uptake mechanism. (3) Promethazine increases NA turnover by enhancing NA release. The discriminative effects of these drugs on the monoamine systems may be related to some differences in their CNS actions. Send offprint requests to K. Saeki at the above address     

The role of dopamine D<Subscript>3</Subscript> compared with D<Subscript>2</Subscript> receptors in the control of locomotor activity: a combined behavioural and neurochemical analysis with novel,selective antagonists in rats

Background The role of dopamine D₃/D₂ receptors in the control of locomotion is poorly understood.Objectives To examine the influence of selective antagonists at D₃ or D₂ receptors on locomotion in rats, alone and in interaction with the preferential D₃ versus D₂ receptor agonist, PD128,907.Methods Affinities of ligands at rat D₂ and cloned, human hD₃, hD_2S, hD_2L and hD₄ sites were determined by standard procedures. Locomotion was monitored automatically in rats pre-habituated for 30 min to an open-field environment. Extracellular levels of dopamine (DA) were determined by dialysis in the nucleus accumbens and striatum. Drugs were given acutely via the systemic route.Results PD128,907, which preferentially recognised D₃ versus D₂ sites, biphasically reduced and enhanced locomotion at low (0.01–0.63 mg/kg) and high (2.5–10 mg/kg) doses, respectively. L741,626 and S23199, which behaved as preferential D₂ versus D₃ receptor antagonists, enhanced the reduction in locomotion evoked by the low dose of PD128,907, blocked the increase provoked by the high dose and suppressed spontaneous locomotion alone. Analogous findings were obtained with haloperidol and raclopride which showed equilibrated affinity at D₂ and D₃ receptors. UH232 and AJ76, which showed a mild preference for D₃ versus D₂ sites, did not modify the effect of a low dose of PD128,907, slightly enhanced the hyperlocomotion elicited by the high dose and exerted little influence on locomotion alone. S14297 and U99194, which acted as preferential D₃ versus D₂ receptor antagonists, abolished the reduction in locomotion elicited by a low dose of PD128,907, potentiated the induction of locomotion by a high dose, and failed to influence locomotion alone. The actions of S14297 were stereoselective inasmuch as they were mimicked by the racemic form, S11566, but not by the inactive enantiomer, S17777. In contrast to S14297, S11566 and U99194, however, S33084, SB269,652, GR218,231 and N-[-4-[-(1-naphtyl)piperazine-1-yl]butyl] anthracene-2-carboxamide (NGB-1), highly selective D₃ versus D₂ receptor antagonists, were inactive under all conditions. PD128,907 (0.01–10.0 mg/kg) suppressed dialysate levels of DA in the nucleus accumbens and striatum, actions blocked by L741,626 and haloperidol, yet unaffected by S14297 and S33084.Conclusions The facilitatory influence of a high dose of PD128,907 upon locomotion is mediated by postsynaptic D₂ receptors and, possibly, countered by their D₃ counterparts. Correspondingly, selective blockade of D₂ but not of D₃ receptors alone suppresses motor function. The reduction in locomotion provoked by a low dose of PD128,907 may be mediated by D₂ autoreceptors, but a role of postsynaptic D₃ receptors cannot be excluded. Finally, mechanisms underlying the contrasting influence of chemically diverse D₃ receptor antagonists upon locomotion remain to be elucidated.     

Independent mediation of unconditioned motor behavior by striatal D1 and D2 receptors in rats depleted of dopamine as neonates

The effects of systemic administration of DA receptor antagonists suggest that unconditioned motor behavior in rats depleted of DA as neonates continues to be dependent upon dopaminergic transmission, yet the specific contribution of D₁ and D₂ receptors to these behaviors has been altered. The purpose of the present study was to determine whether these depletion-induced receptor changes are occurring at the level of striatal DA terminals and their targets. The ability of bilateral intrastriatal injections (0.5 µl) of DA receptor antagonists to induce motoric deficits was determined in adult rats treated with vehicle or 6-OHDA (100 µg, intraventricular) on postnatal day 3. Administration of the D₁-like antagonist SCH 23390 (0.5–2.0 µg) or the D₂-like antagonist clebopride (1.0–4.0 µg) induced dose-dependent akinesia, catalepsy, and somatosensory neglect in vehicle-treated controls. In contrast, neither antagonist produced deficits in rats depleted of forebrain DA as neonates. However,combined administration of SCH 23390+clebopride induced similar akinesia, catalepsy, and somatosensory neglect in both controls and DA depleted animals. Animals depleted of DA were more sensitive than controls to the low doses of this combined D₁+D₂ antagonism. These results demonstrate that activation of striatal DA receptors remains necessary for unconditioned motor behavior in rats depleted of DA as neonates. However, the specific contributions of D₁- and D₂-like receptors to these behaviors differ between intact animals and those depleted of DA as neonates. The ability of endogenous DA acting at either D₁ or D₂ receptors to support spontaneous motor behavior in rats depleted of DA as neonates may contribute to their relative sparing from parkinsonian deficits.     

Radioligand binding and autoradiographic analysis of dopamine receptors in the human heart

Summary The effects of dopamine on the 3-5-cyclic adenosine monophosphate (cAMP) generating system were analyzed in membrane particles from the human right and left cardiac ventricle. In addition, the pharmacological profile and the anatomical localization of dopamine receptors were assessed on frozen sections of human cardiac atrial or ventricular tissue. Dopamine increased cAMP levels, in a concentration-dependent manner, in membranes of the right and the left ventricle. These effects were abolished by the -adrenoceptor antagonist (–)-propranolol, but not by the D₁ receptor antagonist SCH 23390 or by the non selective D₁/D₂ receptor antagonist haloperidol.No specific binding of the D₁ receptor antagonist [³H]-SCH 23390 was noticeable within the atrial or ventricular portions of the heart examined using either radioligand binding or autoradiographic techniques. The D₂ receptor antagonist [³H]-spiroperidol, in the presence of concentrations of ketanserin sufficient to block possible binding to 5-HT₂ sites, was specifically bound to sections of human heart with a dissociation constant value of about 2.6 nmol/l. The highest density of [³H]-spiroperidol binding occurred in the right ventricle followed, in descending order, by the right atrium, the upper part of the left ventricle, the lower part of the left ventricle, the left atrium and the interventricular septum. The binding profile of [³H]-spiroperidol to sections of human heart was consistent with the labeling of dopamine D₂ sites. Light microscope autoradiography revealed silver grains throughout the atrial and ventricular walls and these were frequently accumulated in clusters. These findings suggest that the cardiac actions of dopamine are mediated through the activation of -adrenoceptors and of dopamine D₂ receptors but not of D₁ receptors.     

Dopamine and serotonin metabolites in rat cerebroventricular fluid following withdrawal of haloperidol or electroshock treatment

Levels of the dopamine metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) and of the major serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) were determined in the CSF of rats at various times after repeated electroshock treatment (EST) or chronic administration of haloperidol. The acidic metabolites were analyzed in 25 l CSF using HPLC with an electrochemical detector. A significant decrease in the CSF levels of DOPAC and HVA was found 4 days after the last administration of chronic haloperidol, EST, or both. The decrease in the level of the dopamine metabolites indicated a slower dopamine turnover, which might have resulted from hypersensitivity of presynaptic dopamine receptors after these treatments. Rats treated with haloperidol also showed an increase in 5-HIAA levels, possibly due to enhanced serotonin turnover. The 5-HIAA increase following haloperidol was prevented by a concurrent administration of EST, suggesting attenuation by EST of the haloperidol-induced enhancement of serotonin turnover.     

Morphine and naltrexone modulate D2 but not D1 receptor induced motor behavior in MPTP-lesioned monkeys

Interactions at the behavioral level between dopamine (DA) and opioid receptors in the mammalian brain have been amply demonstrated. Considering the pivotal role for DA receptors in the pharmacotherapy of Parkinson's disease (PD), these interactions might be clinically relevant. Therefore, in the present study the effects of the opioid antagonist naltrexone and agonist morphine on D₁ and D₂ receptor induced stimulation of motor behavior in the unilateral MPTP monkey model (n=5) of PD were investigated. The results show that both naltrexone and morphine [0.1–1.0 mg/kg; intramuscular injection (IM)] inhibited D₂ receptor stimulated contralateral rotational behavior and hand use induced by administration of quinpirole (LY 171555; 0.01 mg/kg, IM) in a dose-related way. However, no effects of these opioid drugs were observed on D₁ receptor stimulated contralateral rotational behavior and hand use induced by administration of SKF 81297 (0.3 mg/kg, IM). Interestingly, the action of the alleged preferential-receptor antagonist naltrexone was mimicked by the selective-opioid antagonist naltrindole (0.5 mg/kg, IM). From this study it is concluded that in a non-human primate model of PD, alteration of opioid tonus leads to modulation of D₂ receptor but not D₁ receptor controlled motor behavior. The possible underlying mechanisms and clinical relevance of these findings are discussed.     

Activation of dopamine D1 receptors does not affect D2 receptor-mediated inhibition of acetylcholine release in rabbit striatum

Summary The possible involvement of dopamine D₁ receptors in the regulation of acetylcholine release in the rabbit caudate nucleus was investigated. Caudate slices, preincubated with [³H]choline, were superfused continuously and subjected to electrical field stimulation with only a single pulse. In agreement with the view that the release of acetylcholine evoked by a single electrical pulse is not influenced by endogenous transmitters, atropine and domperidone failed to icnrease the evoked release of [³H]acetylcholine, whereas oxotremorine and quinpirole caused a concentration-dependent inhibition of transmitter release. Neither the dopamine D₁ receptor antagonist SCH 23390 nor the Dt agonist SKF 38393 in a concentration range of 0.01–1 mol/l changed the evoked [³H]acetylcholine release. The inhibitory effect of the dopamine D₂ receptor agonist quinpirole was virtually abolished in the presence of 0.1 mol/l domperidone and diminished in the presence of 1 mol/l SCH 23390. It remained unchanged in the presence of 1 mol/l SKF 38393. It is concluded that the inhibition of acetylcholine release by dopamine is mediated exclusively via presynaptic dopamine D₂ receptors and that the antagonistic effect of SCH 23390 on the inhibition of acetylcholine release by quinpirole is due to its interaction with dopamine D₂ rather than D₁ receptors located on cholinergic nerve terminals. Send offprint requests to C. Allgaier at the above address     

Occupancy of dopamine D2 receptors by the atypical antipsychotic drugs risperidone and olanzapine: theoretical implications

Rationale To examine the D₂ occupancy of two commonly used antipsychotic medications and relate this to the D₂ occupancy by endogenous dopamine in schizophrenia.Objectives The aim of this study is to compare the occupancy of striatal D₂ receptors by the atypical antipsychotic medications risperidone and olanzapine at fixed dosages and to estimate the effect on D₂ occupancy by dopamine as a result of these treatments.Methods Seven patients with schizophrenia taking risperidone 6 mg/day and nine patients with schizophrenia taking olanzapine 10 mg/day underwent an [¹²³I]IBZM SPECT scan after 3 weeks of treatment. The specific to non-specific equilibrium partition coefficient (V₃) after bolus plus constant infusion of the tracer was calculated as [(striatal activity)/(cerebellar activity)]–1. D₂ receptor occupancy was calculated by comparing V₃ measured in treated patients to an age-corrected V₃ value derived from a group of untreated patients with schizophrenia, previously published, according to the following formula: OCC=1–(V₃ treated/V₃ drug free).Results V₃ was significantly lower in risperidone treated patients compared with olanzapine treated patients (0.23±0.06 versus 0.34±0.08, P=0.01), which translated to a significantly larger occupancy in schizophrenic patients treated with risperidone compared to olanzapine (69±8% versus 55±11%, P=0.01). Data from our previous study were used to calculate the occupancy of striatal D₂ receptors by antipsychotic medications required to reduce the occupancy of these receptors by endogenous dopamine to control values. In medication-free patients with schizophrenia, the occupancy of striatal D₂ receptors by endogenous dopamine is estimated at 15.8%. In healthy controls, the occupancy of striatal D₂ receptors by dopamine is estimated at 8.8%. In order to reduce the dopamine occupancy of striatal D₂ receptors in patients with schizophrenia to control values, 48% receptor occupancy by antipsychotic medications is required.Conclusions These data indicate that the dosage of these medications, found to be effective in the treatment of schizophrenia, reduces DA stimulation of D₂ receptors to levels slightly lower than those found in unmedicated healthy subjects.     

Long-term effects of postnatal amphetamine treatment on striatal protein kinase A activity, dopamine D1-like and D2-like binding sites, and dopamine content

The purpose of the present study was to determine whether exposure to amphetamine during the preweanling period would alter dopaminergic functioning in the dorsal striatum of adult rats. In three experiments, we assessed the effects of repeated amphetamine treatment on striatal protein kinase A (PKA) activity, dopamine (DA) D₁-like and D₂-like binding sites, and DA content. Rats were pretreated with saline or amphetamine (2.5 mg/kg, ip) for 7 consecutive days starting on postnatal day (PD) 11. At PD 90, rats were killed and their dorsal striata (i.e., caudate–putamen) were removed and frozen until time of assay. Amphetamine pretreatment produced long-term reductions in both striatal PKA activity and DA content. Early amphetamine exposure also resulted in an upregulation of D₂-like binding sites, while leaving D₁-like binding sites unaffected. It is likely that the upregulation of D₂-like binding sites was stimulated by the persistent decline in striatal DA levels. Although speculative, it is possible that excess striatal D₂-like receptors were responsible for inhibiting PKA activity through actions on the cAMP signal transduction pathway. The behavioral relevance of these amphetamine-induced neurochemical changes has not yet be determined.     

Differential anti-parkinsonian effects of benzazepine D1 dopamine agonists with varying efficacies in the MPTP-treated common marmoset

In common marmosets systemically treated with MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), the behavioural effects of benzazepine D₁ dopamine (DA) agonists with full/supramaximal (SKF 80723 and SKF 82958), partial (SKF 38393, SKF 75670 and SKF 83565) and no efficacies (SKF 83959) in stimulating adenylate cyclase (AC) activity were investigated. The benzazepine derivatives, with the exception of SKF 82958 (8 fold D₁ DA receptor selectivity), demonstrated high D₁ DA receptor affinity and selectivity (approximately 100 fold or more) in rat striatal homogenates. Administration of MPTP in marmosets induced locomotor hypoactivity, rigidity and motor disability. SKF 38393 (7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine) and SKF 75670 (3-CH₃ analogue) further reduced locomotor activity (by –70 to –80%) and increased motor disability (by +22 to +67%) in these animals. SKF 83565 (6-Cl, 3-CH₃, 3-Cl analogue) and SKF 82958 (6-Cl, 3-C₃H₅ analogue) had only a slight effect on locomotor activity but decreased motor disability at high doses (–46 to –60%). In contrast, SKF 83959 (6-Cl, 3-CH₃, 3-CH₃ analogue) and SKF 80723 (6-Br analogue) produced pronounced increases in locomotion (6–10 fold) and a reversal in motor disability (by –64 to –77%). Oral activity, consisting largely of abnormal, dyskinetic tongue protrusions and vacuous chews, was increased in animals treated with SKF 38393, SKF 83565, SKF 82958 and more especially with SKF 80723 and SKF 83959. Grooming was increased with SKF 82958 and more especially with SKF 80723 and SKF 83959. In contrast, quinpirole (D₂ DA agonist), reversed the MPTP-induced motor deficits in the marmoset, with no effect on grooming and oral activity. The present findings further demonstrate the antiparkinsonian actions of some D₁ DA agonists in MPTP-treated primates. However, in general the behavioural effects of benzazepines failed to correlate with either their D₁ DA receptor affinity/selectivity or their efficacy in stimulating adenylate cyclase (AC) activity. These observations further implicate a behavioural role for D₁ DA receptors uncoupled to AC and/or a role for extrastriatal D₁ DA receptors in mediating the behavioural response to D₁ DA agonists.     

Functional regulation by dopamine receptors of serotonin release from the rat hippocampus: in vivo microdialysis study

The functional regulation by dopamine (DA) receptors of serotonin (5-HT) release from the rat hippocampus was investigated by use of in vivo microdialysis. Dialysate 5-HT levels were reduced by co-perfusion of 10 M tetrodotoxin (TTX) and were elicited by K⁺ (60 and 120 mM) stimulation in a concentration-dependent manner. Local perfusion (10 M) and peripheral administration (20 mg/kg, i.p.) of fluoxetine produced increases in 5-HT levels. These results indicate that the spontaneous 5-HT levels in the rat hippocampus can be used as indices of neuronal origin from the serotonergic nerve terminals. The nonselective dopamine (DA) receptor agonist apomorphine (1, 10 and 100 M), when perfused through the probe over a period of 40 min, increased 5-HT release in a concentration-dependent manner. Apomorphine-induced (100 M) increases in 5-HT release was abolished by pretreatment with the selective D₂ receptor antagonist, S(–)-sulphide (1 and 10 M), but not prevented by pretreatment with the selective D₁ receptor antagonist, R(+)-SCH-23390 (R(+)-7-chloro-8-hydroxy-3-methyl-l-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine) (1 M). S(–)-Sulpiride and R(+)-SCH-23390 by themselves did not alter the spontaneous 5-HT levels. The 5-HT release was elevated by perfusion of the selective DA reuptake inhibitor GBR 12909 (1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-[3-phenylpropyl]piperazine)(1, 10 and 100 M), indicating the possibility of not only exogenous but also endogenous DA-mediated facilitatory effects on 5-HT release in vivo. The 5-HT release was also elevated by perfused (±)-PPHT ((±)-2-(N-phenylethyl-N-propyl)-amino-5-hydroxytetralin)(1, 10 and 100 M), the selective D₂ receptor agonist, in a concentration-dependent manner. On the other hand, (±)-PPHT (100 M) failed to increase 5-HT release in catecholamine (CA)-lesioned rats pretreated with 6-hydroxydopamine (6-OHDA)(200 g/rat, i.c.v.). The (±)-PPHT-induced (100 M) increase in 5-HT release was prevented not only by pretreatment with 10 M S(–)-sulphide but also by pretreatment with the ₂-adrenoceptor antagonist idazoxan (10 M). These findings suggest that the functional regulation of 5-HT release via D₂ receptors exists in the rat hippocampus. Furthermore our results indicate that the facilitatory effect of 5-HT release via D₂ receptors may be mediated indirectly by noradrenergic neurons, but not mediated directly through D₂ receptors located on serotonergic nerve terminals.     

Receptor-Specific Modulation of Risk-Based Decision Making by Nucleus Accumbens Dopamine

The nucleus accumbens (NAc) serves as an integral node within cortico-limbic circuitry that regulates various forms of cost–benefit decision making. The dopamine (DA) system has also been implicated in enabling organisms to overcome a variety of costs to obtain more valuable rewards. However, it remains unclear how DA activity within the NAc may regulate decision making involving reward uncertainty. This study investigated the contribution of different DA receptor subtypes in the NAc to risk-based decision making, assessed with a probabilistic discounting task. In well-trained rats, D₁ receptor blockade with SCH 23 390 decreased preference for larger, uncertain rewards, which was associated with enhanced negative-feedback sensitivity (ie, an increased tendency to select a smaller/certain option after an unrewarded risky choice). Treatment with a D₁ agonist (SKF 81 297) optimized decision making, increasing choice of the risky option when reward probability was high, and decreasing preference under low probability conditions. In stark contrast, neither blockade of NAc D₂ receptors with eticlopride, nor stimulation of these receptors with quinpirole or bromocriptine influenced risky choice. In comparison, infusion of the D₃-preferring agonist PD 128 907 decreased reward sensitivity and risky choice. Collectively, these results show that mesoaccumbens DA refines risk–reward decision biases via dissociable mechanisms recruiting D₁ and D₃, but not D₂ receptors. D₁ receptor activity mitigates the effect of reward omissions on subsequent choices to promote selection of reward options that may have greater long-term utility, whereas excessive D₃ receptor activity blunts the impact that larger/uncertain rewards have in promoting riskier choices.     

Discriminative stimulus properties of cocaine: modulation by dopamine D1 receptors in the nucleus accumbens

Dopamine (DA) D₁ and D₂ receptors are involved in mediating the behavioral effects of cocaine, including its discriminative stimulus properties. The purpose of the present study was to investigate the role of the nucleus accumbens and, in particular, accum bens DA D₁ receptors in modulating the stimulus effects of cocaine. Thus, rats were trained to discriminate cocaine (10 mg/kg, IP) from saline using a two-lever, water-reinforced FR 20 drug discrimination task. In substitution tests, systemic (IP) administration of cocaine (0.625–20 mg/kg) produced a dose-related increase in cocaine-appropriate responding. Microinjections of cocaine (2.5–40 µg) into the nucleus accumbens also engendered dose-dependent and complete substitutions (> 80% drug-lever responding) for the systemic training dose of cocaine, whereas intra-accumbens artificial cerebrospinal fluid (1 µl/side) produced primarily saline-appropriate responding. In antagonism tests, pretreatment with the DA D₁ antagonist SCH 23390 (3–12 µg/kg) completely antagonized (<20% drug-lever responding) a dose of cocaine (5 mg/kg) that produced greater than 90% cocaine-lever responding when given alone. Additionally, intra-accumbens injections of SCH 23390 (0.025–0.4 µg) prior to systemic cocaine (5 mg/kg) also significantly blocked the cocaine stimulus. The present results confirm the importance of the nucleus accumbens in mediating the discriminative stimulus properties of cocaine and suggest a primary role of accumbens DA D₁ receptors in modulating this behavior.Some of these data were presented at the annual FASEB Experimental Biology meeting in New Orleans (1993)     

Increased dopamine receptor sensitivity in the rat following acute administration of sufentanil,U50,488H and d-Ala2-d-Leu5-enkephalin

Summary The effects of acutely administered opioid receptor agonists sufentanil, U50,488H and [d-Ala², d-Leu⁵]-enkephalin (DADL) were observed upon dopamine D₁ and D₂ binding site density in the striatum of the rat. In addition, the functional implications of opioid-induced changes in dopamine receptor sensitivity were studied using the behavioural profile elicited by apomorphine in the rat. The -agonist sufentanil (1 or 20 Erg/kg, i. p.), the -agonist U50,488H (10 mg/kg, i. p.) and DADL (1 g/animal, i. c. v.) all significantly elevated D₂ but not D₁ binding site density in rat striatum. Pretreatment with sufentanil (1 g/kg, i. p.) induced an elevation in apomorphine-induced sterotyped behaviour, but attenuated locomotor activity. Following administration of U50,488H (10 mg/kg, i. p.), both the degree of stereotypy and the intensity of the locomotor activity were enhanced. Contralateral rotation was observed in animals pretreated with DADL (1 g/animal, i. c. v.) following challenge with apomorphine. It is concluded that the opioid agonists studied induce a significant elevation in functional D₂ sites to the exclusion of D₁ sites. However, the precise mechanism by which this effect is elicited remains to be established. Send offprint requests to R. D. E. Sewell at the above address